A wavelength multiplexer/de-multiplexer is used in a wavelength division multiplexing (WDM) communication.
Related techniques are disclosed in, for example, Japanese Laid-Open Patent Publication No. 2002-169054, Japanese Laid-Open Patent Publication No. 2014-182224, Japanese Laid-Open Patent Publication No. 10-335693, Japanese Laid-Open Patent Publication No. 2002-072010, Japanese Laid-Open Patent Publication No. 61-285413, and Japanese Laid-Open Patent Publication No. 2000-028851.